Ominous as that “illegal” moniker sounds, many shoe companies embraced the unsanctioned status of their chunky shoes. After all, the vast majority of runners (hi!) will never set a record or break a finishing-line tape. Popular super shoes like the Adidas Adizero Prime X 2 Strung, Saucony Kinvara Pro, and Hoka Skyward X all are technically “illegal” in that sense—and a soon-to-debut Nike shoe tips the scale even further, rocking a massive 55 mm of cushioning.
Although they’re pricey—up to $500—these shoes are fast, springy, and honestly super fun to run in. As a result, many runners have embraced them not only for their races, but also their day-to-day training miles.
But race-day shoes weren’t meant to be daily trainers.
Despite how nice that turbo boost of speed looks on Strava, there are a few reasons you might want to be cautious about taking these “illegal” shoes out on a daily basis. (Note: Other super shoes, while still cushiony and carbon-plated, like the Saucony Endorphin Pro and New Balance FuelCell SuperComp Elite v4 fall under the World Athletic Association’s parameters, so they’re fair game in races, but they still can stress your body in similar ways—after all, 40 mm isn’t some kind of magical, injury-reducing boundary.)
How super-cushioned shoes can affect your body
When you’ve got a huge amount of foam under your feet, it’s easy to run with sloppy form, University of South Florida professor Irene Davis, PhD, PT, one of the top experts in running biomechanics, tells SELF. “It teaches you to land hard because the more cushioning you have, the less control you need,” she says.
Think of it this way: “If you jump as high as you can and land on the ground, you’re automatically going to bend your hips and knees to cushion the impact. But if you jump on a trampoline, it’s soft, so you can jump with stiffer knees and hips,” podiatrist Alicia Canzenese, DPM, past president of the American Academy of Podiatric Sports Medicine, tells SELF.
Once you get used to that stiffer, less controlled style of running, when you do run in a shoe without so much foam, you can more easily injure yourself. Another tricky part is that the delicate foams used in most performance shoes today are not so durable—while brands usually say regular running shoes typically last from 300 to 500 miles, one ultra-light racing shoe from Adidas was designed for little more than one 26.2-mile marathon.
“As it deteriorates and you’ve trained yourself to land hard, you can end up with impact-related injuries,” Dr. Davis says. Think: plantar fasciitis or knee pain.
Many überthick soles are also just plain unstable. Dr. Hoogkamer points out that to add stack height while keeping a shoe as light as possible, designers will often cut out parts of the foam sole. And while an elite runner might have their form dialed in enough to safely get away with that, average folks might end up with some biomechanical problems—namely, overpronation. “And that puts loads on the plantar fascia, the medial structures of the foot, and all the way up to the knee,” Dr. Davis says.
How the carbon fiber plate can affect your body
Sure, not all “illegally” cushioned shoes include a plate, but many designed for race day do. Since carbon-plated super shoes, sanctioned or not, have only been on the market since 2017, we don’t yet have enough research to know how they’re changing our injury risk, Adam Tenforde, MD, director of running medicine at the Harvard-affiliated Spaulding National Running Center, tells SELF. So far, there’s only one small case series—led by Dr. Tenforde—of five runners who ended up with navicular bone stress injuries at the top of the arch. (FWIW, both Dr. Tenforde and Dr. Davis say that navicular stress fractures are some of the worst injuries runners can get, since they require months off from running to heal.)